The present invention relates to data communication systems and more particularly to a spread spectrum communication system terminal including sequence spreading and frequency hopping.
Spread spectrum communication systems have been used in a variety of fields. In the communication system of this type, the transmitted bandwidth is much greater than the bandwidth or rate of the information to be transmitted. The carrier signal is modulated by some other function to widen or spread the bandwidth for transmission. The received signal is remapped into the original information bandwidth to reproduce a desired signal. The spread spectrum communication system has many useful advantages: a selective call is possible; since the power spectrum density is low, private communication is allowed; and it is little influenced by interference either due to multipath fading or jamming. From this standpoint, the spread spectrum system has found many uses, such as mobile communication systems, avionic systems, satellite communications, scatter communication systems of both the ionospheric and tropospheric type, direction finders and distance measuring equipment.
The spread spectrum systems can be categorized into a direct sequence system, a frequency hopping system, a time hopping system and a hybrid system which is a proper combination of the systems just mentioned. Of these systems, the frequency hopping systems is frequently used in the field of mobile communication systems with a low traffic volume for a number of stations. Also frequency hopping systems can be employed in satellite communication systems and scatter type communication systems where a fading environment is present.
In the frequency hopping system a carrier frequency is shifted or jumped in discreet increments in a pattern dictated by a prepared code of sequence, for instance, a PN (psudo-noise), and M-sequence codes, Gold codes and the like, in synchronism with a change in the state of the codes. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hopping frequency is called a chip. The transmitted information is embedded in the codes or embedded in each frequency of the carrier wave by a so-called FSK (frequency shift keyed) modulation. The information signal thus spread-spectrum-modulated can be reproduced at the receiver.
In reproducing the information signal at the receiver, a synchronization acquisition process is first performed, in which the code pattern provided in the receiver is made accurately coincident with the code pattern generated in the transmitter in time position. Then, the spread spectrum signal is despread, and thereafter a well known demodulation is performed to extract the desired information. More particularly, a local reference signal of a frequency correspondingly determined by the same code pattern as that in the transmitter for every chip and the received signal are mixed in a mixer in order to perform a correlation (despreading) process for converting the spread spectrum signal into the signal having a frequency bandwidth wide enough to extract the information. This system is described in detail in "Spread Spectrum Systems" by R. C. Dixon published by John Wiley and Sons, Inc. in 1976. Following this despreading process, the desired information is extracted by usual demodulation techniques.
It is also known to employ a direct sequence system again employing a PN code, M-sequence codes, Gold codes and the like to spread the transmitted information over the bandwidth of the system and to again employ correlation technique at the receiver to recover the information.
Such systems are not only useful in obtaining a proper coherent transmission in a fading environment, such as is present in mobile communication, satellite communications and scatter communication, the systems are also jammer resistant.
High frequency radio data communication systems have gained increasing importance in recent years where it is used for long distance radio contact as a backup or adjunct to satellite networks. As the high frequency band (2-30 megahertz (MHZ)) is more widely used, the need for reliable high frequency communication links increases. To improve the performance of data modems in terms of bit error rates, recovery from fade dropouts, intersymbol interface due to multipath dispersion effects, and immunity to interference state of the art equalization and error correction techniques must be applied. In addition, spread spectrum signal processing techniques are required to provide antijam capability and low probability of intercept capability.